Modern agriculture, including silviculture, often requires the planting of large numbers of substantially identical plants genetically tailored to grow optimally in a particular locale or to possess certain other desirable traits. Production of new plants by sexual reproduction, which yields botanic seeds, can be slow and is often subject to genetic recombinational events resulting in variable traits in the progeny. Also, such crossing is time- and labor-intensive. Further, inbred strains such as those used to perform such crosses often lack vigor, resulting in low seed productivity.
Despite the drawbacks of conventional crossbreeding by sexual means, botanic seeds produced by such methods have an important advantage in that each seed comprises food-storage organs and protective structures that shelter the plant embryo inside the seed from the harsh soil environment and nurture the embryo during the critical stages of sowing and germination. Without such organs and structures, the plant embryo would be incapable of surviving in nature until it grew to seedling size. Moreover, a botanic seed can survive for long periods of time, often for several years, until conditions are favorable for germination.
In view of the disadvantages of producing large numbers of identical progeny plants by sexual means, propagation of commercially valuable plants via culturing of somatic or zygotic plant embryos has been intensively studied. Such "asexual" propagation has been shown for some species to yield large numbers of genetically identical embryos each having the capacity to develop into a normal plant. Unfortunately, these embryos, which are produced under laboratory conditions, lack the protective and nutritive structures found in botanic seeds. As a result, the embryos must usually be further cultured under laboratory conditions until they reach an autotrophic "seedling" state characterized by an ability to produce their own food via photosynthesis, resist desiccation, produce roots able to penetrate soil, and fend off soil microorganisms. Such extensive laboratory culture during several distinct stages in plant development is time-consuming, resource-intensive, and requires skilled labor.
Some researchers have experimented with the production of "artificial" seeds (i.e., "seed analogs" or, preferably, "manufactured seeds") in which individual plant somatic or zygotic embryos are encapsulated in a hydrated gel. This method evolved from research showing that encapsulating natural seeds in hydrated gels can improve germination in some species.
The hydrated gels used for producing manufactured seeds rapidly lose water to the ambient air or soil on sowing and fail to provide optimal protection to the embryo from mechanical damage resulting from handling and mechanical sowing and from attack by various plant pathogens, herbivores, or other pests, either before or after germination.
In agriculture, natural seed is normally sown during a relatively short period of the biological year. In order to plant large quantities of manufactured seed in this relatively short sowing period, either extremely rapid manufacturing procedures must be developed or the seeds must be stored and thereby accumulated as they are manufactured. It would be possible to develop equipment for manufacturing seed at a speed that would support sowing large crops without long-term seed storage, but seed storage would make the production of manufactured seed more efficient. It is therefore desirable to develop methods that would allow accumulation and long-term storage of manufactured seed. It is also desirable to develop manufactured seeds capable of extended storage without substantial loss of viability before sowing. The present invention meets these and other needs.